This happens to me a lot. The cause, for me, is that while the flight is loading, I go and do something else and Windows "loses focus" on FSX. This was not fixed with a clean reinstall of FSX. It still continues to happen and it almost always happens if I start FSX directly from a saved flight situation and avoiding the start-up screen. I do sometimes see it even starting FSX the normal way, but it is rare. I'm not sure if this is a solution when you get the black screen. I've never had the black screen, just a blank screen.

My remedy is to:
1) Make sure that FSX is the the program that Windows is focused on (CTRLALT+TAB until FSX is selected).
2) ALT+ENTER to bring up the FSX screen. If you use FSX in Full screen mode, then this will make it Window Mode. Press ALT+ENTER again to return it to Full Screen.

Hope this helps.

Mark